Electrical detection or transmission systems typically employ two lines or wires to deliver electrical power to the detector, alarm or other unit requiring electrical power. One of these lines is typically the higher voltage or "hot" line, and the other is the neutral, reference or ground line. One such system is disclosed in Tice et al., U.S. Pat. No. 4,916,432, assigned to the assignee of the present invention, and incorporated herein by reference.
Each set of two lines from a power source may be used to transmit power using electrical pulses, as in the Tice et al. patent, to many detectors or alarms which, for example, may be used throughout an entire floor or section of a large building. In addition, each set of the lines may also be used simultaneously to transmit information, such as the occurrence and location of a fire, between the detectors and a central monitor or control panel.
During the use and operation of these detection or transmission systems, however, faults may develop in the detectors or other equipment which create a short circuit in the transmission lines. These short circuit faults may disable and render inoperative entire sections of the detection system, particularly those sections operating from a common set or pair of transmission lines.
The disabling of such systems, such as fire detection systems, may place otherwise monitored areas at risk pending systems repairs. Accordingly, to avoid such potentially unacceptable risks, a need has arisen for circuits capable of both detecting such short circuit faults and isolating them from the remainder of the system, which may then continue to function normally.
Electrical circuits are currently available which provide some means for detecting and isolating such short circuits. For example, Payne U.S. Pat. No. 4,528,610 discloses an electrical circuit, normally operating in an on state, which switches off in response to a short circuit.
Transistors are used in the transmission line which are normally on and which conduct power or information signals between adjacent transmission line segments. In the event of a short circuit, the transistors no longer have sufficient voltage to be biased in an on state and switch off, isolating the short circuit.
While such known circuits are capable of isolating a short circuit, the known circuitry requires an appreciable electrical current, which may cause significant voltage drops in the line, both during normal operation and in the absence of a short circuit. These current requirements and successive voltage losses limit the number of isolators which may be employed in such a transmission system.
Accordingly, it would be desirable to have short circuit detectors and isolators having minimal current requirements during normal operation and which do not cause successive voltage losses from each detector and isolator unit employed across the lines. In addition, it would be desirable to have such a detector and isolator unit require fewer components, both for ease for manufacture and lower cost to the consumer or other end-user. Thus, there continues to be a need for short circuit detectors and isolators for transmission and detection systems which provide features and benefits of the type described above not heretofore available in such products.